In 1987, a disease syndrome characterized by reproductive and respiratory symptoms in pigs was first described in the United States (1). Late-term abortions, increased numbers of stillborn and weak pigs, poor conception in breeding herds, along with respiratory distress and high mortality in suckling, weaned, and grow-finish pigs was observed (2,3). Similar outbreaks were also described in Europe in 1990 (4). In 1991, the etiologic agent was identified as an enveloped, single-stranded RNA (ssRNA) virus and tentatively classified in the Arteriviridae family, which includes lactate dehydrogenase-elevating virus, equine arteritis virus, and simian hemorrhagic fever virus (2,5,6). The porcine reproductive and respiratory syndrome virus (PRRSV) or Lelystad virus (the European PRRSV isolate) has a genome comprised of seven open reading frames (ORFs). ORFs 1a and 1b; ORFs 2-6 and ORF 7 are predicted to encode for the RNA polymerase, viral membrane associated proteins, and nucleocapsid, respectively (7).